Twisted morphologies and novel chiral macroporous films from the self-assembly of optically active helical polyphosphazene block copolymers

摘要

A series of optically active helical polyphosphazene block copolymers of general formula R-[N=P(O_2C_(20)H_(12))] _n-b-[N=PMePh]_m (R-7 a-c) was synthesized and characterized. The polymers were prepared by sequential living cationic polycondensation of N-silylphosphoranimines using the mono-end-capped initiator [Ph_3P=N=PCl_3][PCl_6] (5) and exhibit a low polydispersity index (ca.1.3). The temperature dependence of the specific optical activity ([α]D) of R-7 a,b relative to that for the homopolymers R-[N=P(O_2C_(20)H_(12))]_n (R-8 a) and the R/S analogues (R/S-7 a,b), revealed that the binaphthoxy-phosphazene segments induce a preferential helical conformation in the [N=PMePh] blocks through a "sergeant-and-soldiers" mechanism, an effect that is unprecedented in polyphosphazenes. The self-assembly of drop-cast thin films of the chiral block copolymer R-7 b (bearing a long chiral and rigid R-[N=P(O _2C_(20)H_(12))] segment) evidenced a transfer of helicity mechanism, leading to the formation of twisted morphologies (twisted "pearl necklace"), not observed in the nonchiral R/S-7 b. The chiral R-7 a and the nonchiral R/S-7 a, self-assemble by a nondirected morphology reconstruction process into regular-shaped macroporous films with chiral-rich areas close to edge of the pore. This is the first nontemplate self-assembly route to chiral macroporous polymeric films with pore size larger than 50nm. The solvent annealing (THF) of these films leads to the formation of regular spherical nanostructures (ca.50nm), a rare example of nanospheres exclusively formed by synthetic helical polymers. With military precision: The first chiral polyphosphazene block copolymers have been prepared. Helical induction (sergeants-and-soldiers effect) explains the enhanced chiral activity of these materials. Self-assembly in thin films revealed the presence of inorganic twisted morphologies, chiral macropores films, and nanospheres formed by chiral block copolymers.